In typical polymer electrolyte membrane (PEM) fuel cells, a membrane electrode assembly (MEA) is disposed between two electrically conductive separator plates. Oxidant and fuel flow fields provide means for directing the oxidant and fuel to respective electrocatalyst layers of the MEA, specifically, to an anode on the fuel side and to a cathode on the oxidant side of the MEA. A typical reactant fluid flow field has at least one fluid channel between an inlet and an outlet in which a fluid stream flows therethrough. The fluid flow field is typically integrated with the separator plate by locating a plurality of open-faced channels on the faces of the separator plate. The open-faced channels face an electrode surface, where the reactants are electrochemically reacted. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide structural support for the electrodes.
It is standard industry practice to flow oxidant air through the cathode fuel flow plate at a higher flow rate than theoretically required for the electrochemical reaction to take place. In other words, the air stoichiometry ratio in the fuel cell is greater than 1, and is typically in the range of about 2. The fuel cell is operated at such an air stoichiometry ratio to, inter alia, avoid localized or systemic oxygen starvation within the fuel cell. Oxygen starvation is a complicated phenomenon that occurs when the partial pressure of oxygen falls below a critical level at one or more locations within the cathode flow field. Effects of oxygen starvation can be observed as a rapid decrease in cell voltage which in severe cases can cause a hot spot on the surface of the membrane.
As air flows along the flow field from inlet to outlet, the oxygen content in the air stream tends to be depleted and the air pressure tends to drop, resulting in reduced performance in the fuel cell. Applicant's own application PCT CA 02/00816 discloses a cathode flow field having delineating flow channels that continuously taper in width from inlet to outlet. The taper is straight, and results in an increased flow rate and reduced pressure drop as the air flows from inlet to outlet. The increasing air flow rate at a given position in the flow channel compensates somewhat for the depletion of oxygen within the air at that position caused by the electrochemical reaction. Therefore, the oxygen availability at a given position in the tapered channel is greater than in a flow channel having a constant cross-sectional area.
While the straight-tapered channels of the PCT CA 02/00816 application do provide improved oxygen availability at the downstream end of the flow field, they do not achieve substantially constant oxygen availability throughout the flow field. It is theorized that maintaining constant oxygen availability throughout the flow field contributes to achieving even current density throughout the fuel cell active area.